Multicuvette rotor for analyzer

ABSTRACT

The invention relates to an analytical rotor comprising a set of analytical cells disposed at its periphery, a central distribution chamber, portioning cavities corresponding to the respective cells and disposed between the cells and the chamber periphery, the cavities each having an inlet connected to the chamber and an outlet connected to one of the cells by a transfer passage, and at least one overflow reservoir connected to the inlets of the cavities by at least one communication passage.

This is a division of application Ser. No. 259,346, filed May 1, 1981,now U.S. Pat. No. 4,431,606.

BACKGROUND OF THE INVENTION

In clinical chemistry, analyses are conducted on very small samples, sothat the accuracy of the results depends closely on the accuracy of theamounts of sample and reagent portions delivered to the area where eachis being conducted. This accuracy of amounts depends on the pipettesused to deliver the portions and is correspondingly difficult toguarantee when very small portions of materials are utilized. This hasresulted in the development of high-accuracy pipettes but thesehigh-accuracy pipettes are very expensive. In addtion, the pipettingoperation has to be repeated for each analyzed sample and each portionof reagent, with the result that the process of preparing a set ofsamples and reagents for analysis in a rotor of a centrifugal analyzerusually takes longer than the analysis itself.

Another essential factor in the accuracy of analysis is thedetermination of the time at which the reaction begins, i.e. when thesample is placed in the presence of the reagent. This is the reason whythe cells are disposed at the periphery of the analytical rotor, whichhas chambers for introducing reagent and/or a sample and communicatingwith the respective cells, the contents of the chambers beingsimultaneously distributed to the cells by the centrifugal force appliedto the liquids upon centrifugation of the rotor.

In U.S. Pat. No. 3,873,217 it has already been proposed to measure outand distribute the portions of the required liquid simultaneously toanalytical cells disposed at the rotor periphery. The measuring-out isperformed as follows: A liquid injected into a central chamber isdivided among various analytical units by equidistant edges or ridges atthe chamber periphery. As a result of the rotation of the rotor and thedivision of the liquid by the equidistant edges, the liquid is forciblyintroduced into the units. The aforementioned proportioning lacksaccuracy and is dependent inter alia on the amount of liquid introducedinto the central chamber, so that the exact quantity has to be pipetted.

According to U.S. Pat. No. 3,744,975, portioning similar to thatdescribed in the preceding patent specification is carried out by usingan overflow reservoir. The portions are simultaneously distributed tothe cells by sending a stream of air under pressure. This does notensure that a very accurate volume of liquid is transferred.

The aim of the invention is to obviate the disadvantages of theaforementioned methods by providing a high-accuracy portioning ofsamples and/or reagents followed by simultaneous distribution of theportions, using a device having a simple structure and adapted to bemass-produced at a reasonable price.

The analytical rotor according to the invention is characterized in thatliquid-retaining means are associated with the transfer passages toprevent liquid flowing into the analytical cells until the centrifugalforce applied to the liquid exceeds a threshold value which is madehigher than the hydraulic resistance to flow through the communicationpassage.

The invention also relates to use of the analytical rotor, characterizedin that a volume of liquid greater than the total volume of theportioning cavities is introduced into the distribution chamber, therotor is driven at a first speed to exert on the liquid a centrifugalforce greater than the aforementioned hydraulic resistance to flowthrough the communication passage but smaller than the threshold value,in order to fill the portioning cavities and discharge the surplusliquid into the overflow reservoir, and the rotor speed is changed to asecond speed greater than the first, in order to exert on the liquid acentrifugal force greater than the threshold value, so as to transferthe portions contained in the portioning cavities to the respectiveanalytical cells.

The accompanying drawings are diagrammatic illustrations, by way ofexample, of various embodiments and variants or the analytical rotoraccording to the invention. In the drawings:

FIG. 1 is a partly cut-away plan view of a first embodiment;

FIG. 2 is a view in section along line II--II of FIG. 1;

FIG. 3 is a plan view of a second embodiment;

FIG. 4 is a view in section along line IV--IV of FIG. 3;

FIGS. 5 and 6 are plan views of two variants of FIG. 3;

FIG. 7 is a view in section along line VII--VII of FIG. 6, and

FIG. 8 is a perspective view of a third embodiment.

FIGS. 9 and 10 are views in section of a fourth embodiment.

FIGS. 1 and 2 show an analytical rotor in the form of a disc 1 having aset of regularly distributed analytical cells 2 at its periphery. Thecentral region of the analytical rotor is used as a distribution chamber3, axially communicating with the exterior via a central aperture 4formed in the top surface 5 of the rotor. The bottom of the distributionchamber 3 is in the form of a dish having a flat central region and anannular frusto-conical edge. The periphery of the distribution chamber 3communicates with a set of triangular portioning cavities 6. All thecavities are separated from one another by ridges or edges 7 formed bythe intersection of two adjacent sides of each cavity. All the ridges 7are arranged circumferentially about the distribution chamber. Therespective bases of the triangles, adjacent the circle, open towards thedistribution chamber 3. The apex of each triangle opposite its open basehas an outlet aperture at the end of a transfer passage 9 connectingeach portioning cavity 6 to an analytical cell 2. As shown in FIG. 2,the edge formed by the apex of the triangular cavity 6 adjacent passage9 forms an angle smaller than 180° and the end of the transfer passage 9and the apex lie along the same radius. This configuration facilitatesemptying cavities 6 under the action of centrifugal force. The transferpassages 9 comprise capillary ducts, the cross-section of which ischosen so that the resulting capillary forces maintain a cohesivemeniscus of liquid at the outlet of the transfer passage 9. To this end,each passage 9 opens into a surface of cells 2 parallel to the axis ofrotation of rotor 1 and half-way from the top and bottom surface of thecells, in order to substantially reduce the risk of destabilization ofthe resulting meniscus.

The inlets of the portioning cavities 6 are adjacent an annular wall 10which covers them and whose top surface is configured as an inclinedplane, starting from an edge 11 of wall 10 adjacent to the inlets 8. Theinclined plane terminates in an annular collector 12, the bottom ofwhich has apertures 13 giving access to triangular overflow reservoirs14 each inserted between two adjacent portioning cavities 6. Thecros-ssections of the respective apertures 13 are made sufficientlylarge to prevent capillary force being exerted on the liquid flowingthrough them.

Each cell 2 comprises two windows 15 disposed opposite one another alongan axis parallel to the axis of rotation of rotor 1. The windows are forphotometrically measuring the contents of the cells.

Rotors 1 are made by injection-moulding of transparent plastics. In theexample, each part of rotor 1 on either side of a central plane M isseparately injection-moulded and the same applies to the top surface 5.The material used is transparent to UV, and in the present case isnon-stabilized methyl polymethacrylate, sold by ICI under the registeredtrade-mark DIAKON. The various injection-moulded parts are preferablycured before being joined together, using pure solvents such aschloroform of dichloroethane. The parts can also be assembled byultrasonic welding.

Rotor 1 is used as follows: It is placed on the driving plate of ananalytical apparatus (not shown). A certain volume of reagent isintroduced into the distribution chamber 3 and the rotor is rotated at afirst speed of the order of 400 to 600 rpm for 4 to 8 s. At this speed,the liquid in the distribution chamber 3 is a ejected by centrifugalforce into the portioning cavities 6 and travels by capillary actionthrough the various transfer passages 9 up to their ends, which openinto the vertical walls of the respective analytical cells 2, where astable meniscus forms and thus prevents the air enclosed in cells 2 fromflowing out. Since cells 2 are also hermetically sealed, thethus-imprisoned air prevents liquid centering cells 2, since thecentrifugal force communicated to the liquid by the rotor at theaforementioned speed of 400-600 rpm is insufficient to overcome theresistance of the volume of air imprisoned in cells 2. The amount ofreagent introduced into the distribution chamber 3 has deliberately beenmade greater than the total volume of the portioning cavities 6, and theresult is that an excess of liquid remains after the cavities have beenfilled. The excess is then expelled along the inclined plane of the topsurface of annular wall 10 into the annular collector 12, the bottom ofwhich is formed with apertures 13 allowing the liquid to flow into thevarious overflow reservoirs 14. As a result, only the portionscorresponding to the volumes of cavities 6 remain at the periphery ofchamber 3, and the overflow means ensures that the entire volume ofcavities 6 is filled as cavity 6 is sized to assure that the quality ofreagent corresponds to that required, the measured volumes are veryaccurate.

In a second step, the speed of rotor 1 is rapidly increased to4,000-5,000 rpm for 2-5 seconds. At this speed, the centrifugal forceexerted on the portions of liquid retained in cavities 6 is sufficientfor the pressure of the liquid to break the meniscus and thus allow theair to escape from cells 2 by passage 9, so that the liquid canprogressively enter the cells, droplets of incoming liquid alternatingwith outgoing air bubbles until all the liquid has been transferred fromcavities 6 to cells 2, the transfer occurring simultaneously for all thecells.

If the sample has already been introduced into the cells, the reactionsbegin and measurement by any usual techniue can be made via windows 15after reducing the rotor speed to 400-600 rpm in order to measurevariations in the absorbance of the samples during their reactions,using a well-known method.

If, on the other hand, the samples are not yet in cells 2, they can beintroduced into cavities 6 by a pipette. In this case, the volume of thesample must be less than the volume the portioning cavity can contain atrest, since otherwise the various samples would come into contact andmix with each other.

After the samples have been introduced into the portioning cavities,they are simultaneously centrifuged at 4,000-5,000 rpm and driven intothe respective cells, and measurement is begun after reducing the rotorspeed to 400-600 rpm.

The design of the described analytical rotor is entirely based onretaining the liquid between the portioning chambers 6 and the cells 2,owing to the capillary effect of passages 9, which maintain a cohesivemeniscus which in cooperation with the air in the hermitically-sealedcells, prevents air escaping, with the result that liquid cannot beintroduced into the cells except by applying a differential pressuresufficient to break the meniscus. On the other hand, the passage forexcess liquid into the overflow reservoirs offers considerably lowerresistance. Consequently, when cavities 6 are filled up to theirrespective inlets, the excess liquid is conveyed to reservoirs 14 alongthe inclined plane of wall 10, the annular duct 12 and the apertures 13,which are dimensioned so that air can escape and liquid cansimultaneously enter. Liquid is transferred from the portioning cavities6 to the cells 2 after increasing the pressure of the liquid as a resultof the centrifugal force produced by raising the rotor speed from400-600 rpm to 4,000-5,000 rpm. This method of transfer communicatesforce to each portion of liquid and guarantees the complete distributionof liquid in cells 2, thus ensuring that the measurements made in thecells are accurate. By contrast, this result cannot be ensured bytransfer in a flow of fluid under pressure, as proposed in prior art. Inaddition, centrifuging is the conventional method of transfer in thiskind of analysis. Of course, other means, different from thosedescribed, could be devised for retaining the liquid in the portioningcavities. However, the enormous advantage of the described retainingmeans is that they are completely static and are obtained only bysuitably dimensioning the various parts of the rotor. The rotor can thusbe obtained by injection and sticking or welding, i.e. at a pricecomparable with known analytical rotors. It can be used on existingcentrifugal analytical devices and only requires a low-accuracy pipettefor introducing the portion of reagent into the distribution chamber 3.

FIGS. 3 and 4 show an embodiment which differs from the preceding mainlyin that the analytical rotor 16 has only two, diametrically opposite,overflow reservoirs 17. The portioning cavities 6 and the analyticalcells 2 are in every respect similar to those in the precedingembodiment. Consequently, we shall give a detailed description only ofthe structure of the overflow reservoirs 17, which are connected to thecentral distribution chamber 3 by respective communicating passages 18at the ends of recesses 19 each bounded by two of the edges 7 at theintersection between the adjacent sides of the two portioning cavities 6and the edges of each recess 19, so that the edges of recesses 19 are atthe same radial distance from the axis of rotor as the inlets of thevarious cavities 6.

The interior of the overflow reservoirs 17 is connected to two adjacentlateral chambers 20 near the inlet of the passage 18 into each reservoir17. Chambers 20 are connected to the external atmosphere by apertures 21formed through the top surface of rotor 16. As a result, the resistanceto liquid entering reservoir 17 is lower than at the inlet of cells 2,since reservoirs 17 communicate with atmosphere via apertures 21 whereasthe cells are hermetically sealed as soon as liquid blocks the transferpassages 9.

During low-speed centrifuging (400-600 rpm), the reagent introduced intothe chamber 3 is driven into cavities 6 and recesses 19. Since two flowsoccur through passages 18 and all the edges 7 are along a single radius,the excess liquid inside the radius through edges 7 flows towardsrecesses 19 and into reservoirs 17.

The air volume corresponding to the volume of the introduced liquidescapes freely to atmosphere via apertures 21. The barriers extend fromone edge to the other of the reservoir, leaving an intermediate passage22a. They are adapted to prevent liquid flow back during the subsequentoperations of mixing the liquid reagent and the sample, by oscillationof the analytical rotor.

FIG. 5 shows a variant, the only difference from the embodiment in FIGS.3 and 4 being the presence of circular grooves 23 formed in the bottomand top wall of chamber 3 and adjacent edges 7. The grooves are adaptedto help the meniscus to stick in the inlet 8 of the proportioningcavities 6, thus improving the portioning accuracy.

FIGS. 6 and 7 illustrate another variant of FIGS. 3 and 4. In additionto grooves 23, the variant comprises a pair of ribs 24 disposed in linewith each side of cavities 6 adjacent the recesses, the ribs beingformed at the bottom of chamber 3. Ribs 24 are adapted to slow down theflow of liquid towards recesses 19. The recesses can modify the meniscusat inlets 8 of the adjacent cavities 6 and thus slightly distort theportions, but this is prevented by ribs 24. It must be remembered thatthe aim of the invention is to attain accuracy of the order of 1% oreven below, so that any source of error must be eliminated.

Tests performed with the described rotors show that these make itpossible to attain the aforementioned accuracy and reduce the errorfactor below 1%, thus equalling the accuracy of the best known pipettes.In addition, the method of portioning is not affected by any faults inoperation, since the portioning does not result from the action of anymoving part and the rotors are used only once and come from a singlemould.

The rotors described in connection with FIGS. 3-7 are made byinjection-moulding a top and bottom part having substantially equalthickness and stuck or welded by ultra-sound as previously explained.

FIG. 8 shows an embodiment of a rotor 25 especially designed forbacteriological analysis. Rotor 25 has analytical cells 26 connected toa portioning cavity 27 by passages 28. The centre of the rotor isoccupied by a distribution chamber 29 towards which all the inlets ofthe portioning cavities 27 extend. An overflow reservoir 30 occupies theposition of an analytical cell 26 and is connected to the distributionchamber 29 by three passages 31, two of which open into chamber 29 viarespective recesses 32 at the ends of the passages, so as to collect thesurplus liquid and enable it to flow through them, whereas the duct 31which does not open into a similar recess enables air to escape fromreservoir 30 towards the distribution chamber 29 after the excess liquidhas entered the reservoir. This feature avoids any contact with theexterior, which is important in the case of bacteriological analysis. Onthe other hand, the air leaving duct 31 forms foam which may slightlyreduce accuracy. In the case, however, of bacterilogical analysis,accuracy is less important than the risk of contamination.

The top surface of rotor 25 is covered by a stuck or welded plate 33. Asbefore, the plate has an analysis window 34 coinciding with a similarwindow formed in the bottom of each cell 26. Plate 33 also has apertures35, e.g. for introducing an antibiotic product for testing in thepresence of a given bacterial strain. A closure means 36 is provided forhermetically closing cells 26 after the product for testing has beeninserted. The product is preferably placed on a holder in the form of ahydrophilic paper disc of the kind sold under the registered trade mark"Sensi-disc" by Becton, Dickinson & Co. A culture broth is introducedinto the distribution chamber 29 in the same manner as the reagent inthe preceding embodiments, and is centrifuged at a first speed in orderto fill the portioning cavities 27. The excess liquid is guided byrecesses 32 into passages 31, whereas the air comes out by the passage31 without a recess 32. Once all the excess liquid has been transferredto the overflow chamber 30, the speed of rotor 25 is increased as beforein order to drive the portions of liquid into cells 26. Cell 37 (besidechamber 30) is used as reference cell.

Alternatively, the rotors can be sold after filling the cells with givenantibiotic products, thus relieving the user of this task. The presenceof the transfer passages 28 adjacent plate 33 reduces the stability ofthe meniscus at the outlets of the aforementioned passages, butsimplifies the manufacture of rotor 25, which can be made in one pieceby injection-moulding. In this case, an ordinary plate 33 formed withapertures 35 and a window is sufficient to close the top of the assemblyof cells 26 and chambers 27. The accuracy obtained with thelast-mentioned embodiment isquite compatible with the accuracy requiredfor bacteriological analysis. In the case of a pre-loaded rotor, theapertures 35 can be eliminated, in which case the cells are filledbefore plate 33 is welded or stuck on.

The embodiment shown in FIGS. 9 and 10 essentially differs from theembodiments described above on the one hand in the disposition of theoverflow reservoir, and on the other hand in the disposition of thedistribution chamber with respect to the other parts of the rotor.

The rotor shown in FIG. 9 consists of a distribution chamber 42 which isapt to receive the liquid to be portioned and distributed bycentrifugation and of an assembly 41 comprising portioning cavities 6,transfer passages 9, analytical cells 2, an annular overflow reservoir44 disposed at the lower part of the assembly and a communicationpassage 43 leading to the overflow reservoir. This passage comprises anannular collector 43 formed and disposed below the portioning cavities.This collector has an inlet 47 and is separated from the portioningcavities by an annular wall 45, which extends from the lower edge of theinlets 46 of said cavities to the overflow reservoir.

As shown in FIG. 9 and 10, with respect to the assembly 41, the centraldistribution chamber 42 can take two positions along the rotational axisof the rotor:

a first position, shown in FIG. 9, in which the radially outermost upperedge 48 of said chamber is at a level comprised between the levels ofthe upper and the lower edges of the inlets 46 of the portioningcavities 6; and a second position in which said upper edge of thechamber 42 is at a level comprised between the levels of the upper andthe lower edges of the inlets 47 of the annular collector 43.

The change from the first position to the second can be obtained bydisplacing the chamber 42 with respect to the assembly 41 or vice versa.

The rotor shown in FIGS. 9 and 10 is used as follows: It is placed onthe driving plate of an analytical apparatus (not shown). A volume ofliquid greater than the total volume of the portioning cavities 6 isintroduced into the central distribution chamber 42. With this chamberin its first portion (shown in FIG. 9), the rotor is rotated at a firstspeed of about 600 rpm for 4 to 8 s. At this speed, the liquid in thedistribution chamber 42 is ejected by centrifugal force into theportioning cavities 6 and travels by capillary action through thevarious transfer passages 9 up to their ends, which open into thevertical walls of the respective analytical cells 2, where a stablemeniscus forms and thus prevents the air enclosed in cells 2 fromflowing out. Since cells 2 are also hermetically sealed, thethus-imprisoned air prevents liquid entering cells 2, since thecentrifugal force communicated to the liquid by the rotor at theaforementioned speed of about 600 rpm is insufficient to overcome theresistance of the volume of air imprisoned in cells 2. As the amount ofreagent introduced into the distribution chamber 3 has deliberately beenmade greater than the total volume of the portioning cavities 6, anexcess of liquid remains after these cavities have been filled. Theexcess is then expelled into the annular collector 43 which allows theliquid to flow into the overflow reservoir 44. Consequently, only theportions corresponding to the volumes of cavities 6 remain in thatcavities. In this way, the overflow means ensures that the entire volumeof cavities 6 is filled, with the result that the measured volumes arevery accurate. In a further step, the chamber 42 is put in the secondposition (shown in FIG. 10) and the speed of the rotor is rapidlyincreased to 4,000-5,000 rpm for 2-5 seconds. At this speed, thecentrifugal force exerted on the portions of liquid retained in cavities6 is sufficient for the pressure of the liquid to break the meniscus andthus allow the air to escape from cells 2 by passage 9, so that theliquid can progressively enter the cells, droplets of incoming liquidalternating with outgoing air bubbles until all the liquid has beentransferred from cavities 6 to cells 2, the transfer occurringsimultaneously for all the cells.

If the sample has already been introduced into the cells, the reactionsbegin and measurement can be made via windows 15 after reducing therotor speed to about 600 rpm in order to measure variations in theabsorbance of the samples during their reactions, using a well-knownmethod.

The embodiment described above with reference to FIGS. 9 and 10 isparticularly advantageous when due to the properties of the liquidtreated, it can be expected that some drops of liquid remain adhered tothe central receptacle of the distribution chamber at the end of theinitial centrifugation of the rotor at about 600 rpm. While such dropswould be erratically expelled into the analytic cells during thetransfer step (centrifugation at about 4,000-5,000 rpm) in theembodiments described above with reference to FIGS. 1-8, this cannothappen in the embodiments described with reference to FIGS. 9-10, sincein this embodiments such excess drops are certainly expelled into theoverflow reservoir during the transfer step. A similar effect can alsobe obtained without having to provide the second position (shown in FIG.10) of chamber 42, i.e. leaving it in the first position during thetransfer step, provided that during the initial centrifugation step atabout 600 rpm the chamber 42 is rotated at a greater speed than theassembly 41. In this alternative embodiment the chamber 42 rotates e.g.at about 1,000-1,500 rpm during the initial centrifugation step, whilethe assembly 41 is rotated at about 600 rpm. During the transfer step,the chamber 42 remains in the position shown in FIG. 9 and is preferablyat rest, while the assembly 41 is rotated at about 4,000 rpm in order totransfer the liquid portions from the portioning cavities to therespective analytic cells.

In a preferred embodiment of the rotor shown in FIGS. 9 and 10, therotor chamber 42 has an annular cover 49, e.g. a disc with a circularopening in the middle and attached to the assembly 41, and lateralapertures 50 with a diameter of about 0.5 mm uniformly disposed alongthe periphery of the chamber. These features assure an even more uniformdistribution of liquid to the portioning cavities 6. A thin slit betweencover 49 and chamber 42 can replace the apertures 50.

The aforementioned invention can be used for simultaneously supplying anumber of portions, with an accuracy within 1%. Consequently, theportioning means not only replaces the most accurate pipettes but savesconsiderably time compared with the use of a pipette to load the rotor.The invention, therefore, saves investment by replacing the precisionpipette and increases productivity as a result of the simultaneousportioning. These advantages have hitherto been associated with variousdisadvantages which particularly affected accuracy, inter alia in thecase of clinical chemical analyses. The main advantage of the inventionis to solve the problem of portioning without using a pipette but withrigorous accuracy comparable with that of the most accurate pipettes, byusing static means resulting from the design of the rotor which, likeknown rotors, can be made in two or three injection-moulded pieces,which are assembled by sticking or welding. Consequently the analyticalrotors can be mass-produced at a price which is completely competitivewith other analytical rotors which do not incorporate portioning means.

We claim:
 1. A method for transferring by centrifugal force apre-determined quantity of a liquid to an analytical cell which formspart of an analytical rotor wherein said rotor has a plurality ofanalytical cells located around the periphery of the rotor, each cellhaving only one aperture which is normally open and is the inlet of thecell, a central distribution chamber, and a plurality of concentricallylocated portioning cavities, each portioning cavity being adjacent tothe central distribution chamber and an analytical cell, wherein theportioning cavity is located between the outer periphery of the centraldistribution chamber and the analytical cell, each portioning cavitybeing sized to receive the pre-determined quantity of liquid and havingan inlet which connects it to the central distribution chamber and anoutlet which connects it to the aperture, in the analytical cell whichit is adjacent to, via a capilary passageway which opens into a verticalsidewall of the analytical cell, and the rotor further having at leastone overflow reservoir for receiving excess liquid connected to thecentral distribution chamber by at least one connecting path, saidmethod comprising the steps of:(a) introducing a given quantity ofliquid greater than the total volume of all the portioning cavities intothe distribution chamber of the rotor; (b) as a result of driving saidrotor on a centrifuge at a first rotational speed to exert centrifugalforce on the liquid introduced into the central distribution chamber,transferring the pre-determined quantity of liquid into each portioningcavity and into each capillary passageway, forming a meniscus at the endof each capillary passageway where it opens into the vertical wall ofthe analytical cell which it is connected to hermetically sealing theanalytical cell with the meniscus so that air within the analytical cellcan no longer flow out thereby creating a resistance to flow of theliquid into the analytical cell, and transfering excess liquid into theoverflow reservoir; and (c) then as a result of driving the rotor at asecond rotational speed greater than the first rotational speed,breaking the meniscus which was formed in step (b) at the end of each ofthe capillary passageways and passing said pre-determined quantity ofliquid from each portioning cavity through each corresponding capillarypassageway into the analytical cell to which it is connected whileallowing air which was trapped in the analytical cells to escape througheach of the capillary passageways.
 2. The method of claim 1 wherein thecapillary passageway of the rotor has a cross-section chosen so thatresulting capillary forces maintain a cohesive meniscus of the liquid atthe outlet of the capillary passageway to the analytical cell when therotor is driven at the first rotational speed and when the rotor isdriven at the second rotational speed, the resulting contrifugal forcebreaks the meniscus and thereby permits the liquid to enter theanalytical cell.
 3. A method in accordance with claim 1, wherein thefirst rotational speed is about 400-600 rpm and the second rotationalspeed is about 4,000-5,000 rpm.